This invention relates to a magnetic recording and playback apparatus capable of carrying out simultaneous monitoring at the time of recording, which is adapted to record Pulse Code Modulation (PCM) signal while obliquely forming tracks on a magnetic tape by means of rotary heads at a predetermined time interval.
There is generally known a spherical scan type magnetic recording and playback apparatus called a Digital Audio Tape recorder (DAT) constituted to sequentially record pulse code modulated audio signals, etc. on a magnetic tape by means of a plurality of rotary magnetic heads.
With DAT, PCM signals obtained by applying pulse code modulation to information to be recorded are recorded on a tape conforming to an Industry Standard. After such PCM signals are subjected to recording signal processing such as time compression, they are recorded while obliquely forming signal tracks on a magnetic tape by means of rotating two magnetic heads with no guard band being formed between tracks.
The above mentioned two magnetic heads used in the recording are such that a head for A channel of plus (+) azimuth (A head) and a head for B channel of minus (-) azimuth (B head) are mounted or affixed and arranged on the rotary drum so as to form an angular interval of 180 degrees to each other. These heads are commonly used for recording and playback. Each head gap width (hw) is 1.5 times larger than the track pitch (Tp).
The recording of PCM signals with the abovedescribed DAT having A and B heads is conducted as follows.
First, according to the Industry Standard and as shown in FIG. 3, a signal track (n-2 track) of the A channel having a track width 1.5 times larger than the track pitch (Tp) is formed by the A head. Then, a signal track (n-1 track) of the B channel having a width 1.5 times larger than the track pitch Tp is newly formed by the B head so as to overwrite the one third (1/3) portion (0.5 times larger than the track pitch (Tp)) of the upper side of the n-2 track. As a result, an n-2 track having a nominal track width which is equal to the track pitch (Tp) and two thirds (2/3) of the track width initially formed, remain formed on the tape.
Then, an n track of A channel having a track width 1.5 times larger than the track pitch (Tp) is newly formed by the A head so as to overwrite the one third (1/3) (0.5 times larger than the track pitch (Tp)) of the upper side of the n-1 track. As a result, a normal n-1 track having a nominal track width equal to the track pitch (Tp) and two thirds (2/3) of the track width initially formed, remain formed on the tape.
At times subsequent thereto, the above-described normal A and B tracks will be alternately formed in succession on the magnetic tape.
The track pattern recorded here includes various kinds of signals in addition to the above-described PCM signal. Namely, as shown in the well known Automatic Track Finding (ATF) track pattern of DAT (e.g., p. 37, the March 1987 issue of "Electronics Life" published by Japan Broadcasting Publishing Association), a pilot signal f.sub.1 (130.67 KHz), timing signals f.sub.2 (522.67 KHz) and f.sub.3 (784.00 KHz), and an erasing signal f.sub.4 (1.568 KHz) are recorded in the longitudinal direction of the track before and after the PCM signal, with it being as a center.
The reproduction or playback of the magnetic tape on which the above-described tracks are recorded, will be conducted as follows.
When A head (B head) traces A track (B track), the A head (B head) senses a crosstalk component of the pilot signal f.sub.1 recorded on the adjacent B track (A track) to generate a tracking error signal (ATF signal) derived from the signal f.sub.1, by using the ATF signal, a rotational phase control of a capstan for transporting the magnetic tape by a predetermined amount to control a degree of transportation of the magnetic tape so that the A head (B head) traces the central portion of the A track (B track). Thus, a reproduced PCM signal with extremely less crosstalk components can be obtained satisfactorily by two magnetic heads with the azimuth effect.
However, when recording the PCM signal, the DAT of the above arrangement is unable to monitor its recording performance while the recording is taking place. One of the conceivable arrangements directed to this problem is that the rotary drum, a total of four rotary magnetic heads of an A channel recording head (first channel recording head), an A channel playback head (first channel playback head), a B channel recording head (second channel recording head), and a B channel playback head (second channel playback head) are positioned at predetermined angular intervals and axial distances along on rotary axis of the rotary drum.
The four magnetic heads used here are the A channel recording head of plus (+) azimuth, the A channel playback head of plus (+) azimuth, the B channel recording head of minus (-) azimuth, and the B channel playback head of minus (-) azimuth. The azimuth angles of the respective channels are different from each other.
FIGS. 1 and 2 are a plan view and a front view showing a sliding contact between the magnetic heads and the magnetic tape in the conceivable magnetic recording and playback apparatus of the above arrangement. FIG. 3 is a view showing the playback head trace pattern of the conceivable magnetic recording and playback apparatus.
As shown in FIG. 1, an A channel recording head (an A recording head) 1, an A channel playback head (an A playback head) 2, a B channel recording head (a B recording head) 3, and a B channel playback head (a B playback head) 4 are mounted onto a rotary drum 5 in clockwise order at an angular interval of 90 degrees between respective heads. When the rotary drum 5 rotates, respective heads 1 to 4 slidably become in contact with a magnetic tape 7 which is wrapped around the rotary drum 5 in an angular range of substantially 90 degree, with being pressed onto the rotary drum 5 by loading poles 6 and 6. In FIG. 1, symbols R and S denote the rotational direction of the rotary drum 5 and the direction of movement of the magnetic tape 7, respectively.
The four magnetic heads 1 to 4 used here, have an azimuthal relationship such that the A recording head 1, the A playback head 2, the B recording head 3 and the B playback head 4 have plus (+), plus (+), minus (-) and minus (-) azimuthal polarities, respectively, wherein the azimuth angles of the respective channels are different from each other. The head gap width hw of each head is 1.5 times larger than the track pitch Tp.
As shown in FIG. 2, the rotary drum 5 is composed of an upper drum (first drum) 8 and a lower drum (second drum) 9, and the A playback head 2 is mounted or affixed on the lower end of the upper drum 8 of the rotary drum 5. The A recording head 1 is affixed at a position spaced higher from this by an axial distance d1. Similarly, the B playback head 4 is affixed on the lower end of the upper drum 8 and the B recording head 3 is affixed at a position spaced higher from this by the axial distance d1.
The axial distance d1 is set as follows. Namely, as shown in FIG. 3, the axial distance between the A recording head 1 and the A playback head 2 is set so that right after the A track (n) is formed the A playback head 2 traces A track (n) covering partially the area thereof limited to lower 2/3, and picking up a recorded signal correspondingly. When this monitoring takes place, the upper 1/3 of the track n is not yet erased or overwritten by the subsequent track n+1 (not shown). This causes an undesirable amplitude fluctuation of the picked up signal because of a fact that the A playback head 2 has an inherent tracing deviation in a transverse direction of track when tracing thereof so that the covered area of the track n by the gap of the A playback head 2 is not always uniform but irregular along the track n.
The same is true with the B playback head 4.
Accordingly, the above-described conceivable magnetic recording and playback apparatus has a capability of simultaneous monitoring of PCM signals recorded on the A and B channel tracks having the normal track pitch Tp, but has the problem of amplitude fluctuation in the monitored signal making the monitoring system less reliable.